The present invention relates to range imaging systems, and more particularly to range imaging systems employing scannerless range imaging techniques.
U.S. Pat. No. 4,935,616 describes a scannerless range imaging (SRI) system using an amplitude-modulated high-power laser diode to completely illuminate a target scene. Conventional optics confine the target beam and image the target onto a receiver. The range to the target is determined by measuring the phase shift of the reflected light from the target relative to the amplitude-modulated carrier phase of the transmitted light. To make this measurement, the gain of an image intensifier within the receiver is modulated at the same frequency as the transmitter, so the amount of light reaching the receiver is a function of the range-dependent phase difference. A second image is then taken without receiver or transmitter modulation and is used to eliminate non-range-carrying intensity information. Both captured images are registered spatially, and a digital processor is used to operate on these two frames to extract range. Consequently, the range associated with each pixel is essentially measured simultaneously across the whole scene.
The device described in the ""616 patent uses a two-dimensional array of detectors (such as a charge-coupled device (CCD) image sensor) that simultaneously captures range information of all of the elements in a two-dimensional projection of a three-dimensional scene. Periodically modulating the illumination source and simultaneously modulating the gain of the receiver accomplish this. The receiver is comprised of a photocathode, which converts incoming photons to a multiplicity of parallel electron streams; a micro-channel plate, which amplifies the electron streams; and a phosphor screen, which converts the electron streams back to visible radiation. The image formed by the phosphor screen is imaged onto the CCD sensor. Modulating the gain of the micro-channel plate causes a modulation of the intensity of the image appearing on the CCD sensor. Beating of the modulation of the light reflected from the object against the modulation of the receiver gain results in an image, each pixel of which has an amplitude that is proportional to the cosine of a phase shift between the reflected light and the receiver modulation. This phase shift in turn is proportional to the range of the corresponding object point. The range of each object point can be computed and a monochromatic range image can be formed wherein the intensity of each pixel in the image is proportional to the range of the corresponding object point from the camera.
The preferred method of estimating the range in the ""616 patent uses a pair of captured images, one image with a destructive interference caused by modulating the image intensifier, and the other with the image intensifier set at a constant voltage. However, a more stable estimation method uses a series of at least three images, each with modulation applied to the image intensifier, as described in commonly assigned copending application Ser. No. 09/342,370, entitled xe2x80x9cMethod and Apparatus for Scannerless Range Image Capture Using Photographic Filmxe2x80x9d and filed Jun. 29, 1999 in the names of Lawrence Allen Ray and Timothy P. Mathers. In that application, the distinguishing feature of each image is that the phase of the image intensifier modulation is unique relative to modulation of the illuminator. If a series of n images are to be collected, then the preferred arrangement is for successive images to have a phase shift of       2    ⁢    π    n
radians (where n is the number of images) from the phase of the previous image. The resultant set of images is referred to as an image bundle. The range at a pixel location is estimated by selecting the intensity of the pixel at that location in each image of the bundle and performing a best fit of a sine wave of one period through the points. The phase of the resulting best-fitted sine wave is then used to estimate the range to the object based upon the wavelength of the illumination frequency.
An image intensifier operates by converting photonic energy into a stream of electrons, amplifying the number of electrons and then converting the electrons back into photonic energy via a phosphor plate. If it is desired to produce a normal brightness image (herein called a texture image) the device described in the ""616 patent can be operated with the modulation to the micro-channel plate turned off. Although both the texture and range images are precisely aligned (due to the common optical path shared by both images), one consequence of this process is that color texture information is lost. Since color is a useful property of images for many applications, a means of acquiring the color information that is registered along with the range information is extremely desirable.
It is possible to use multiple optical pathways in the receiver of a SRI so that a colored texture image and a monochromatic range image can both be formed on a single image sensor. Such an approach is described in detail in commonly assigned copending application Ser. No. 09/572,522, entitled xe2x80x9cMethod and Apparatus for a Color Scannerless Range Image Systemxe2x80x9d and filed May 17, 2000 in the names of Lawrence Allen Ray and Louis R. Gabello, and which is incorporated herein by reference. In this system, a primary optical path is established for directing image light toward a single image responsive element. A modulating element, e.g., a micro-channel plate, is operative in the primary optical path to receive an infrared component of the image light and a modulating signal, and to generate a processed infrared component with phase data indicative of range information. A secondary optical path is introduced, which routes the visible color texture image around the micro-channel plate in the primary optical path. A system of lenses, beamsplitters, and mirrors can be used to form the second optical path, and a shutter can be employed in the second optical path to switch the light on and off in the path. Although this modification would enable the capture of range and colored texture images with a single CCD sensor, it introduces the problem of possible misalignment of the range and texture images. As a consequence, depth information cannot be accurately assigned to each point in the colored texture image.
There is a need therefore for a method whereby the colored texture and range images in a color SRI camera system can be precisely aligned.
The need is met according to the present invention by providing a method of aligning a color scannerless range imaging system of the type having an illumination system for illuminating a scene with modulated infrared illumination, a color image sensor, image forming optics for forming an image of the scene, an optical arrangement for forming first and second optical paths between the image forming optics and the image sensor, and a transponder subject to modulation located in the first optical path for amplifying and converting infrared light to visible light to form a range image on the image sensor. The method includes the steps of providing a target having alignment indicia that can be imaged in both infrared and visible regions of the spectrum, capturing an infrared image of the target using the first optical path, capturing a color image of the target using the second optical path; and adjusting at least one of the optical paths so that the captured images are coincident.
This invention provides a technique whereby the colored texture and monochromatic range images in a SRI system having separate optical paths for the color and range images and a single color image sensor can be precisely aligned.